Add KeyModifiers enum.

config-serializer/config-items/KeystrokeModifiersAction.ts

@@ -1,3 +1,14 @@
+enum KeyModifiers {
+    leftCtrl   = 1 << 0,
+    leftShift  = 1 << 1,
+    leftAlt    = 1 << 2,
+    leftGui    = 1 << 3,
+    rightCtrl  = 1 << 4,
+    rightShift = 1 << 5,
+    rightAlt   = 1 << 6,
+    rightGui   = 1 << 7
+}
+
 class KeystrokeModifiersAction extends KeyAction {
 
     @assertUInt8